1. Field of the Invention
This document relates to a display apparatus, and more particularly, to an apparatus and a method for measuring a polarization direction of a polarizing plate for a liquid crystal display.
2. Discussion of the Related Art
Liquid crystal displays are generally devices displaying an image using optical anisotropy and birefringence properties of liquid crystal molecules.
In a liquid crystal display, two substrates each having electrodes for generating electric field face each other so that the surfaces of the two substrates on which the electrodes are formed are opposite to each other, and a liquid crystal material is injected between the two substrates.
Arrangement of the liquid crystal molecules is changed by an electric field obtained by applying a voltage to the electrodes, and thus the liquid crystal display displays an image by controlling the quantity of light transmitted through the substrates.
A thin film transistor-liquid crystal display (TFT-LCD) using thin film transistors (TFT) as switching devices has been generally used.
In the TFT-LCD, white light generated in a backlight unit passes through liquid crystal pixels such that transmittance of the white light is controlled. Then, a color image is displayed using additive color mixtures of light transmitted through red (R), green (G) and blue (B) color filter layers positioned on the liquid crystal pixels.
A display level of the TFT-LCD is determined by controlling the quantity of transmitted or reflected light depending on the intensity of electric field in each pixel.
Because characteristics of the liquid crystal display depend on the polarization state of a first polarizing plate and a second polarizing plate set in a cross-Nicol configuration, optimum designs of the first polarizing plate and the second polarizing plate are necessary and the polarization directions of the first and second polarizing plates have to be accurately known.
However, when the polarization directions of the first polarizing plate and the second polarizing plate are not accurately controlled, the amount of light passing through a liquid crystal layer of the liquid crystal display is not accurately controlled. This leads to a reduced image quality of the liquid crystal display.
FIG. 1 illustrates a configuration of an apparatus for measuring a polarization direction of a polarizing plate of a related art liquid crystal display.
Referring to FIG. 1, the related art polarization direction measuring apparatus includes a light source 71, a first polarizing plate 72, a sample holder 73, a sample 74, and a light detector 75. Light is generated by the light source 71, and the light detector 75 detects the quantity of light passing through the first polarizing plate 72 and the sample 74.
Referring to FIG. 1, the related art polarization direction measuring apparatus includes the first polarizing plate 72, of which a polarization direction is accurately known, and the sample holder 73 capable of accurately positioning a sample 74 along a reference axis thereof.
In other words, the polarization axis of the sample 74 is inclined relative to the polarization axis of the first polarizing plate 72 at a predetermined angle, and the sample 74 is placed along the reference axis of the sample holder 73. Then, while rotating the sample 74 in the azimuth direction, the light detector 75 detects when the minimum quantity of light is detected. Therefore, the angle (i.e., a polarization direction) of the sample 74 inclined relative to the reference axis is obtained.
The related art polarization direction measuring apparatus can accurately measure the polarization direction of the sample 74 when the polarization direction of the first polarizing plate 72 is accurately known.
Further, the reference axis of the sample holder 73 has to be known accurately to mount the sample 74 into the sample holder 73. In other words, the polarization direction of the first polarizing plate 72 and the reference axis of the sample holder 73 have to be known accurately for accurate measurement of the polarization direction of the sample 74. Accordingly, it is difficult to accurately measure the polarization direction of the sample 74.